1. Technical Field
The present disclosure is directed to electrosurgery and, in particular, to circuitry for measuring or sensing the contact resistance or impedance between the patient and pairs of RF return pad contacts or electrodes employed in such surgery.
2. Description of the Related Art
One potential risk involved in electrosurgery is the possibility of stray electrical currents causing excess heating proximate the RF return pad contacts or patient return electrodes. The most common conditions which are thought to lead to excess heating include:
(1) Tenting: Lifting of the return electrode from the patient due to patient movement or improper application. This situation may lead to excess heating if the area of electrode-patient contact is significantly reduced;
(2) Incorrect Application Site: Application of a return electrode over a highly resistive body location (e.g., excessive adipose tissue, scar tissue, erythema or lesions, excessive hair) will lead to a greater, more rapid temperature increase. Or, if the electrode is not applied to the patient (i.e. electrode hangs freely or is attached to another surface), the current may seek an alternate return path such as the table or monitoring electrodes; and
(3) Gel drying either due to premature opening of the electrode pouch or use of an electrode which has exceeded the recommended shelf life.
Many monitor or detection systems have been developed in the past, but most cannot directly guard against all three of the above listed situations. In order to protect against these potentially hazardous situations, the contact resistance or impedance between the return electrode and the patient should be monitored in addition to the continuity of the patient return circuit.
Safety circuitry is known whereby split (or double) patient electrodes are employed and a DC current (see German Pat. No. 1,139,927, published Nov. 22, 1962) or an AC current (see U.S. Pat. Nos. 3,933,157 and 4,200,104) is passed between the split electrodes to sense the contact resistance or impedance between the patient and the electrodes. U.S. Pat. No. 3,913,583 discloses circuitry for reducing the current passing through the patient depending upon the area of contact of the patient with a solid, patient plate. A saturable reactor is included in the output circuit, the impedance of which varies depending upon the sensed impedance of the contact area.
The above systems are subject to at least one or more of the following shortcomings:    (a) lack of sensitivity or adaptiveness to different physiological characteristics of patients and    (b) susceptibility to electrosurgical current interference when monitoring is continued during electrosurgical activation.
U.S. Pat. Nos. 4,416,276 and 4,416,277 describe a split-patient return electrode monitoring system which is adaptive to different physiological characteristics of patients, and a return electrode monitoring system which has little, if any, susceptibility to electrosurgical current interference when monitoring is continued during electrosurgical activation. The entire contents of both U.S. Pat. Nos. 4,416,276 and 4,416,277 are incorporated herein by reference.
Still a need exists for a detection or monitoring system, which is: 1) adaptive to different physiological characteristics of patients; 2) has little, if any, susceptibility to electrosurgical current interference, (including interference or measurement interaction between components of the detection system); 3) can measure or sense the contact resistance or impedance between the patient and pairs of RF return pads or electrodes where multiple pairs of RF return pads are utilized due to the high current frequently needed during electrosurgery, such as during tissue ablation; and 4) eliminates or minimizes the risk of measurement interaction between the RF return pad pairs.
Therefore, it is an aspect of the invention to provide a multiple RF return pad contact detection system for use during electrosurgical activation which achieves the above objectives.